Wind turbine generators include power converters that are used to provide a required output, which must account for plant operating parameters, grid conditions and instantaneous demand, from generated power, which is variable in dependence on instantaneous operating conditions.
In some power converters, three-phase generated power is fed to an array of switches that are operated using pulse-width modulated (PWM) control signals to provide a required output. The high-frequency switching of the control signals can create audible noise, in particular where harmonics of the PWM input are produced in the audible range.
The characteristics of the acoustic noise generated are dependent on the carrier frequencies of the control signals that are used to operate the switches of the power converter. In turn, the carrier frequencies may be influenced by both the properties of the generated power and the instantaneous output to be delivered.
Under certain operating conditions, the noise produced in the converter may become unacceptable. This is especially the case when the carrier frequency remains steady at a level that tends to produce harmonics at a corresponding constant frequency, which manifest as continuous monotone noise, or ‘tones’, emitted by the converter. Tones are known to be particularly stress-inducing for any individual within close proximity of the converter, and so are undesirable.
Some frequencies generate noise of significantly higher magnitude than is caused by neighbouring frequencies, a phenomenon that is analogous to resonance, and tonal audibility or tonality is a measurement of the amplitude of noise at a particular frequency compared with at its neighbouring frequencies. One way to reduce tonal audibility is to vary the switching frequency of the converter over time to minimise the production of harmonics, although doing so can impact the performance of the converter.
It is against this background that the invention has been devised.